In the production of aluminum by the electrolysis of aluminum oxide (Al.sub.2 O.sub.3), the aluminum oxide is dissolved in a fluoride melt consisting mainly of cryolite (Na.sub.3 AlF.sub.6). The aluminum, which separates out at the cathode, collects below the fluoride melt on the floor of the cell, and the surface of the liquid aluminum forms the cathode of the cell. Anodes, which are made of amorphous carbon in conventional processes, are inserted into the melt from above. As a result of the electrolytic decomposition of the aluminum oxide, oxygen forms at the anodes and combines with the carbon of the anodes to produce CO and CO.sub.2. The temperature range in which the electrolytic reaction takes place is approximately 940.degree. C to 975.degree. C.
The well known principles of the prior art governing a conventional aluminum electrolytic cell, using pre-baked carbon anodes, are illustrated in FIG. 1 of the accompanying drawings, which shows a vertical section along a longitudinal direction through a part of the cell. The fluoride melt 10, i.e. the electrolyte, is held in a steel container 12, which is lined with a thermally-insulating, refractory material 13 and with carbon 11. The aluminum 14 separated out at the cathode is lying on an upper surface 15 of the carbon floor of the cell. The cathode is formed by the surface 16 of the liquid aluminum. Within the carbon floor, and disposed in a direction transverse to the longitudinal direction of the cell, are iron cathode bars 17, which conduct direct current from the carbon lining 11 out of the cell. Anodes 18 made of amorphous carbon are inserted into the fluoride melt 10 from above and supply the electrolyte with direct current. The anodes 18 are connected via conductor rods 19 to the anode bus bar 21, to which the rods 19 are securely fixed by means of clamps 20. The electric current flows from the cathode bars 17 of one cell, through a busbar which is not illustrated here, to the bus bar 21 of the next cell connected in a series. The current flows from the bus bar 21, through the conductor rods 19, the anodes 18, the electrolyte 10, the liquid aluminum 14, and the carbon lining 11 to the cathode bars 17. The electrolyte 10 is covered by a crust 22 of solidified electrolyte, on top of which there is disposed a layer 23 of aluminum oxide. While the cell is in operation spaces 25 are formed between the electrolyte 10 and the crust 22. A crust of solidified electrolyte, i.e. a lateral ledge 24, also forms at the side wall of the carbon lining 11. This lateral ledge 24 determines the horizontal surface area of the bath of liquid aluminum 14 and of the electrolyte 10.
The distance D between the bottom 26 of the anode and the surface 16 of the aluminum, called alternatively the interpolar distance, can be changed by raising or lowering the bus bar 21 by means of jacks 27, which are mounted on columns 28. On operating the jacks 27, all the anodes are raised or lowered at the same time. The anodes can also be set individually at any chosen height by means of the clamps 20 disposed on the bus bar. bar 21.
Oxygen, which is released during electrolysis, reacts with the anodes, with the result that they are consumed at the bottom of the cell at the rate of approximately 1.5 to 2 cm per day, the amount depending on the type of cell. The cell is designed so that, at the same time, the surface of the liquid aluminum in the cell rises by 1.5 to 2 cm per day. After an anode has been consumed in this manner, it is replaced by a new anode. In practice, a cell is operated in such a way that after only a few days, the anodes are consumed in varying degrees, with the result that the anodes have to be replaced individually over a period of several weeks. This results, as is shown in FIG. 1, that anodes at different stages of their respective lifetimes are in operation in any one cell.
In the course of electrolysis the concentration of aluminum oxide in the melt 10 falls. When the concentration of aluminum oxide is as low as 1 - 2 %, an anode effect occurs, i.e. a sudden increase in voltage, from a normal 4 to 4.5 V, to 30 V and higher, is observed. Then the crust must be broken and the concentration of Al.sub.2 O.sub.3 increased by adding more aluminum oxide.
Under normal operating conditions the cell is usually fed fresh Al.sub.2 O.sub.3 at regular intervals, even when no anode effect occurs. In addition, every time the anode effect occurs, as described above, the crust on the cell must be broken and the Al.sub.2 O.sub.3 concentration increased by adding fresh aluminum oxide, which means that the cells must be serviced. In production therefore, an anode effect is always associated with cell service, which in contrast to normal cell maintenance can be described as "anode effect service".
The aluminum 14 produced, which collects on the carbon floor 15 of the cell, is generally removed once a day from the cell by a special device designed for this purpose.
In the course of time a significant increase in volume of the carbon lining 11 occurs due to penetration of components from the electrolyte. The components meant here are either salts, which are contained in the fluoride melt, or chemical compounds which originate from the melt as a result of reactions which at present are not well understood.
As a result of its increase in volume, the carbon lining presses horizontally against the thermal insulation 13 and the peripheral wall of the steel container 12. The steel is consequently irreversibly deformed, which stresses it beyond its elastic limit and therefore can cause cracking. The carbon lining is deformed too, the carbon lining on the floor usually bowing upwards, with the result that cracks form in its upper surface 15. The liquid aluminum 14 then penetrates these cracks and reacts with the iron cathode bars 17. The damage to the lining of the cell can progress so far that the liquid aluminum is discharged from the cell. The cell then has to be taken out of service and the whole lining replaced before the cell can be put back into service. Such repairs are usually necessary every 2 to 6 years; they are expensive and there is also a loss in production due to the cell being out of service.
Attempts have been made, namely by reinforcing the steel container, to prevent this deformation and cracking. However this cannot be entirely prevented, but only reduced in severity. Furthermore such reinforcement is a significant economical disadvantage, since the cell becomes more expensive and its overall weight is considerably increased.
During the time the cell is not operating it is often necessary to undertake expensive repairs on the steel container in order to eliminate deformation and cracks.
A great deal of effort has been exerted towards eliminating the penetration of electrolyte components into the carbon lining, and with the attendant resultant increase in volume. It has been found, however, that this volume change cannot be entirely prevented and therefore must be accepted as unavoidable.